Effects of stress relaxation in beta-titanium orthodontic loops: Part II

Abstract

Objective: To determine which regions of beta-titanium T-loop springs (TLSs) are more affected by the stress relaxation over a 12-week period. Materials and Methods: Fifty TLS were previously activated by concentrated bends and divided into five groups of 10 each according to their evaluation periods: immediate assessment (G0), 24 hours (G1), 48 hours (G2), 1 week (G3), and 12 weeks (G4). Groups 1 to 4 were mounted into a structure simulating a clinical situation. After the experimental periods, the springs were scanned for measurement of their angles and numbered from 1 to 9. A two-way analysis of variance was used to detect differences among the angles measured and differences caused by time and also to detect interactions between those two factors. Tukey's test was used to find differences among the groups. Results: Time influenced the angulations of the TLSs (P , .001). Tukey post hoc test showed that G0 (84.1u) presented a different profile, whereas G1 (90.2u), G2 (90.7u), and G3 (91.1u) had similar profiles among each other, with G4 (92.6u) showing a mean value different from all other groups. A significant interaction was detected between activation time and angular deformation in the TLSs (P , .01). Conclusion: Stress relaxation was observed in the TLSs. It was greatest within 24 hours and gradually increased up to 12 weeks. Two regions were identified as responsible for the relaxation of the TLSs: one at the bend between the vertical extensions of the springs and the base arch and the other at the preactivation bends made in the base arch. (Angle Orthod. 2016;86:386-390.)

Nickel titanium T-loop wire dimensions for en masse retraction

“…Curvature was added rather than concentrated bends to the base wire because it has been shown that they may be less sensitive to stress relaxation overtime. 6,23 The M/F produced by the T-loops was different among the groups, which was our most interesting finding since with linear alloys M/F ratios do not appear to change with increased cross section. 18,24 Our results showed similar moments between the two largest wire sizes, but the 0.018" 3 0.025" T-loop showed higher forces, thus having a lower M/F ratio ( Figures 5 through 7).…”

“…23 That large stress concentration induces a stress-induced martensitic transformation that will make that region more flexible rather than more rigid; moreover, further increasing the angulation in that region will cause a different effect than the desired one because higher stresses will make the load/deflection rate of NiTi even lower. 7,21 A solution for that problem has been shown by Bourauel and colleagues 9 with parallel tubes and stainless steel base wires, but that would still allow stress relaxation to occur in that specific region, because there was a 90u bend in the stainless steel base wire after the tubes.…”

Stability of beta-titanium T-loop springs preactivated by gradual curvature

“…This decrease of force over time is in agreement to several other reports that have measured this effect on straight wires, showing it to be time-dependent. 9,11,13 There were only two studies 14 , 18 that looked upon this effect in more elaborate configurations, evaluating stress relaxation on TLSs preactivated by bends, in which a decrease of 15.5% on the force levels of TLSs was shown in the first 24 hours. The present results also show a decrease over time, but with a different behavior that is probably due to the gradual curvature that was used for preactivation, distributing stress over the entire extent of curvature.…”

Segmental Mushroom loop for individual canine retraction in the maxillary and mandibular arch - A case series

“…Stress relaxation was greatest in T loop springs within 24 hours and gradually increased up to 12 weeks (28). Stress relaxation occurred at the bend between the vertical extensions of the springs and the base arch and the other at the preactivation bends made in the base arch (28).…”